Mixing torpedo for plastics extruders



Aug. 19, 1952 F, E. DULMAGE MIXING TORPEDO FOR PLASTICS ETXTRUDERS 2SHEETS--SHEET l Filed June 28, 1951 INVENTOR. Freder/ck E [2U/mage' Aug-19, 1952 F. E. DULMAGE MIXING ToRPEDo FOR PLASTICS EXTRUDERS 2SHEETS--SHEET 2 Filed June 28, 1951 TOR. /mage INVEN E. Du

Frederick ATTORNEYS.

Patented Aug. 19,..21952 UNITED. STATE-s PATENT Y OFFICE MIXING TORPEDOFOR-PLASTICS Y A' EXTRUDERS' f n Frederick E, Dulmage, Saginaw, Mich.,assignor to The, Dow Chemicall Company, Midland, Mich., a corporation ofDelaware Y Application-June'ZS, 1951, 'Serial No. 233,995 (cl. is-'lm v3 Claims. Y 1

This invention relates to an improvedtorpedo for plastics extrusionmachines, and particularly to a-plasticating, mixing and homogenizingdevice for use in such extruders. A

Various meansl have been used to blend organic thermoplastics with suchmodifying inat'erials as plasticizers, lubricants, pigments or dyes, andthe like. This has often required the use of a plurality of machinessuch as dry mixers, compoundingv rolls,- various types of kneaders ormasticators, and other diverse machines. This is costly,'time consuming,and each handling of the material increases the chances ofcontamination. To overcome this problem, various attempts have been madeto effect the mixingr and homogenizing yin the extrusion device itself.Since most such machines are worm-fed, yand since a feed worm isfundamentally inefcient as armixer, the suggestions have usuallyinvolved complex baffling attachments to create turbulence and mixingwhere none normally occurs. These devices create inordinately large backpressures, and uneconomical amounts of work are required to drive theworm. They do not avoid or overcome the .common pulsations of a screwfedextruder. Further, it has usually been necessary to put the extruded mixthrough the machine two or three times to effect the desired blendingand to realize the uniformity previously obtained with compoundingmills, and each pass through the extruder has had to be followedbyacutting, chopping or grinding step to give a suitable granular feed forthe next pass.r Such multiple-pass treatments may require the'plastic,especially if heat-sensitive, to remain heated long enough to inducedegradation or oxidative discoloration.

It is laccordingly among the objects of the present invention to providea means, operable in conjunction with an extrusion device, for blendinga plastic mix to a state of homogeneity in a single pass through theextruder. A related object is to provide such a means which will notoffer increased resistance to flow of material through the extruder, andhence, which will not increase the powerrequirementsof the extruder. Afurther object is to provide a novel andjeicient mixing torpedo, adaptedto be p used in standard plastics extrusion apparatus. Another object isthe provision of such a mixing torpedo which can be used withoutchanging thebarrel of, -;astandard extrusion machine and with only lsimple changes being required in the Yworm of such machine. yAnadditional object is to modify a screw-fed extruder to avoid or overcomethe usual surges and pulsations in the delivery rate ef suchvmachines.

about 1A; to about 1/2A the length of the cylindrical sleeve of theextruder, the torpedo having a plurality of equally spaced helicalgrooves with rounded bottoms in its cylindrical surface, said grooveshaving the same hand as the feed Worm and having a lead of from 1/2 to 2times the length of the torpedo, there being also provided a pluralityof spacedvneck-like circumferential constrictions along the torpedo, ofthe same depth as the said helical groovesthe constrictions representingabrupt decreases in diameter of the torpedo at their feedends andgradual returns to the original diameter at their discharge or forwardends, the combined capacitiesl of the helical grooves andcircumferential constrictions being atleast as great as the deliverycapacity of the saidcoaxial feed worm. a

Experience has shown that, While the abovedescribed, prior torpedoeffects a greater homogenizing action on plastic dry mixes than has beenpossible before with any single piece of extrusion apparatus, it stillhas certain shortcomings which leave room for improvement in attainingthe general objects of the invention. Thus, the` lands between theround-bottomed grooves of the prior torpedo arev all the same radius andhence provide exactly the same shearing action, or freedom fromshearing, on any lm of plastic on the barrel of the extruder.Consequently, the plastic which is advanced along each groove is mixedto a reasonable state of uniformity within that groove, but may have adifferent composition from that in other grooves as there is negligibleinterchange between ma- -terial in the various grooves. Some `blendingand further homogenizing occurs in the neck-likel constrictions betweenthe grooved sections of the torpedo, but further improvement ispossible.

It is accordingly a further object of the present invention'to provide amixing torpedo yfor use in plastics extruders which is more eicientthan, and is an improvement on, the torpedo of my prior Patent No.2,453,088. Additionalobjects may appear from the following description.A

vThe improved torpedo of the presentinvention y is adapted to be securedcoaxially to the forward end of a shortened worm of an extruder. It fitsof neck-like constrictions along the torpedo rep-.

resenting abrupt decreases in the diameter of the torpedo to the depthof the grooves at their lfeed f end and gradual increases to theoriginalv diameter at their discharge or forward ends. The

new torpedo differs from the prior one, however, in the shape of thegrooves, and in having theY various lands disposed about its peripheryof different radii in certain parts of its length, as well as by havingthe grooves in the successive sections of decreasing depth, as will bedescribed more fully.

In the new torpedo, the grooves do not 'have rounded bottoms, i. e., incross-section they do not'represent sections of a circle. Instead, theleading side of each groove is straighty and lies on 'thera'd'ius of thetorpedo. The groove, in cross-section', resembles the lletter In., withthe saidleading side being the short leg of the L, and thelongertrailingside being arcuate, `with a radius vapproximating that ofthe torpedo and with its center of' curvature lying outside of thetorpedo. The grooves in each: section of the torpedo have the samecircular pitch as those'in every other section, but those in the sectionnearest the feed end of thetorpedo are deeper thanl those in the nextsection, each successive section of the torpedo toward the discharge-end having shallower grooves than the precedin'g'sections. Regardlessof their dilerent depths, the grooves in all sections have the followingidentical features. Their'leading side lies on'a radius of the torpedoandforms the trailingV side 'of one land; their arcuate side extendsfrom a point on that radius toV a' point near the circumference of the.torpedo vwhich is on the leading edge of the next land; the saidarcuateside hasy approximately the same radius of curvature 'as does thetorpedo, but with its center outside the torpedo; and, 'all grooves inall sectoins have the same width on the circumference of the torpedo;

The lands in the discharge section of the torpedo all have the sameradii, and hence the same clearance from the surrounding concentricbarrel. In prior sections of the torpedo, however, the lands are dividedsymmetrically into three' groups'. One group of lands (theV third,sixth, ninth, etc., in the order of rotation) has a greater lradius thanthe others, and thus has the least clearance Within the working barrel.Another groupv (the first, fourth, seventh, etc.) has a'srnallerdiameter than any of the others, andlhas the greatest radial clearancefrom the barrel.' Ihe intermediategroup of lands (the second, iifth,eighth, etc.) has a radius greater than that of the' first describedgroup 'and less than that of the second group described. Thus, if thelands in the discharge section have about 0.006 inch radial clearancefrom the cylindrical barrel, those'inthe other` sections of the torperomay have radial clearances of 0.006 inch in the first group, 0.011 inchin the intermediate group and 0.016 inch in the other group. Thus, inrotation of the torpedo within the barrel, plasticwhich rolls on to thebarrel wall, past 4. one of the lands having greatest clearance, isscraped partially from the wall by the succeeding land havingintermediate clearance, and most of the remainder is scraped from thewall by' the succeeding land having minimum clearance. In this manner,material is transferred from the mass in onegloove ,to v that in othergrooves in the's'ame section of the torpedo, and mixing is augmented bythe amount of such transfer.

The invention will now be described with reference to the accompanyingdrawing, wherein Fig. l is a longitudinal section through part of thelbarrel of a standard plastics extruder, showing the feed worm or screwand the new torpedo, in elevation;

Fig. 2 is an enlarged elevation of the torpedo;

' Fig. 3 is a cross-sectional view of the torpedo taken along line 3-3of Fig. 1;

Fig. 4 is a similar section taken along line 4--4g and Fig. -5' is asimilar section taken along line 5-'-5 of Fig. 1. i Y

The torpedo -I0 ofthe present invention is used in the cylindricalbarrel II of a standard,

usually horizontal, extrusion machine I2, and is a coaxial extension ofa shortened feed screw I3 of conventional'type. Screw I3 usually, butnot necessarily, has a root I4 at least half the diameter of-barrel II,and is ordinarily of the compression type.' with a diminishing lead onsuccessive ights of the screw. Torpedo I0 is suitably from one-quarterto one-half the length of the extruder-:barrel II', from the hopper (notshown) to the head I5. Typically, the barrel II is surrounded by chamberI6, for heating or cooling,` as'required. Beyond torpedo I0, in theextrusion path, is a screen or strainer Il, held in place across the"discharge end of barrel II by head I5, which is secured to the body ofextruder I2 by means of bolts which may be inserted or withdrawn throughbolt holes I8 and tapped sockets I9; Head I5 may be of any desired form,and may be straight (as shown) or of the gooseneck or L varieties. -Anydesired type of extru- Ysion oriiice21'or orices may be used.

as the'direction of rotation of the screw I3 and torpedo III, so as to'maintain a. tight coupling between the two when said stud is turned intoa socket (not shown) provided for that purpose in the forward end of theshortened screw I3. A tapered circumferential shoulder 2I is providedbetween the torpedo body and the root I4 of screw I3, to furnish asmooth approach over which plastic material maybe forwarded as it leavesthe screw. At least one, and usually from two to ve, circumferentialnecks 22 are provided at Aregularly spaced intervals along tor` pedo IU,and eachineck 22 adjoins a tapered shoulder 23 at its forward ordelivery end to furnish la smooth and gentle return from the lesserdiameter ofr neck 22 tor the greaterv body diameter of torpedo I0. Formost efficient operationfthe angle of taper is between about 12v,"Y

and 20, Vthough angles as high as 30 maybe used. Disposed helicallyabout the torpedo I0 is a series of' skewed grooves 2li.A Grooves 24N'i5 furnish. continuous smooth passagewaysy from 91.16 :mithin-tothenext. .'.fFIfhe right .helicoid grooves .24 are of opposite :hand to.the'trotation of the screw I.3 .and torpedo VI,|l,:i. e.;;they` havethe same* hand @as the iiights fofv screw V.I3,. and -haYe-:a lead.lengthzwhich may varyL-fromone- 'half `to two times the length of the,torpedopn the modifica-tion illustrated, grooves :24 have a l thecharacteristics previously defined, ias is shown 1in.y the respectivecross-,sectional views -of Figs. ..3- The grooves 24 in the feed sectionVof tor- 4pedo yIIlare shown inFigf. yThe leading face 30 7.of.,eachgroove `2 ll'lies on ,a radius of the torpedo IU. l The arcuate4face k3| .of eachggroove24 has a .radius R vapproxim ately equal tothat of torpedo I D, orV `of the bar-relII in which lthe torpedo ishoused. In Athe said, iirst -or -feed section, the ralreof Athe Vleadingface 30 `of each groove.24, relative to .a lline normal to vits arcuateface l3I is about,1'5. Similarly, a line normal to arcuate face 3 I ofeachgroove 24forms an-angle of about 28 with the radius of torpedo I0passing through the midlineof the .groove opening. Torpedo I0 'islillustrated as. having twelve grooves andra like `nurnbenoflands. Eachthird land .25 has only vi'vorking Y clearance within barrelu -I I,while the preceding. land, in fthe direction of rotation, has`a..greate'r. clearanceand .the one ahead vofthat has still greatervclearance.within the barrel..

; ..Re'ferring-tolig. the grooves `24. in the center .section of torpedoII) are seen to be .of thesame fopen .widthas those in the feed section.In.the

`center section,`the .difference between grooves 24 iromfthose in the.feed section is that the rake of the leading face .30 of. each groove24, relative to V a line normal tothe arcuate .face 3 I ,lisonlyabout.6l30, while the line normal to said arcuate face 3 I vforms an angle ofabout 19 with .the radius of torpedo II) which passes through-the.midline of u lthe groove...o`pening..Consequently, .grooves 24areshallowerin the centerY section than in the feedgsection. VSuccessivelands 2.5 inthe center section-are. stepped, ask are those in the feedsection.

VFig. 5 illustrates theshape of Agrooves 24 fand..

of lands 25 in the, discharge section of torpedo .I Il'. .In thissection, the grooves 24 vhave the same open; width as. lthose in thepreceding sections, but .areshallower than those in either vof thepreviouslyidiscussed sections of the torpedo.

Thisdiiierence is attained by reducing. the rake of the leading face3010i each groove 24 to 0, and' by reducing the angle .between Va linenormal .to thearcuate-,face 3-I and theradius of the'torpedo whichpasses through theI midline of the groove 'A 60 Whereas the roundedgrooves and uniformly opening to about Y12.a 30'.

Within each groove and because of the partial transfer of material fromone groove to the next due to the graded clearances between the landsand the barrel.

'.In operation,l fthe .elxtruder I2 is llieatedv by Ysteam :in .chamber4.I E and the `Vdesiredirig'reilients ofthe nalmix1are-fed toscrew 113,suitably from or through-a hopper '.(not shown). .The screwla vand itsvcoaxial extension, y'torpedo fIU, arer s etiin motion byv a.motor-drivenvgear (not illustrated) in .the conventional manner. vIntheillustrated modifications, rotation is tothe right-since screw AI 3 yandgrooves .are both "lefthanded; f'- Initial fusion and preliminary,non-'uniform 'mixing occursV as-vthe feed iis advancedfby-the'lscrewtowardthetorpedo. .".When the feed `reaches the torpedo itisadvanced'out'wardly overshoulder! I to-the .entrances 'ltoA grooves' 24,among. whichE it isv evenlydistributed... Examination has shown thatcontinued! rotation. of "itorpedo 10, .when grooves 124 are filled with.plas'tic,.results in' unique agitation .of 'thel mass. Apparently,'friction'between Ithe barrel I I and the plastic in `groovesizl'lcauses that plastic :to .ro'll within'thegroovaboth axially of thegroove, with .a leftward rotation, and forwardly. along V.the groove.The.' massfin each .groove becomes :thoroughly 'rn'ixed "There iskcontinuous exchange .between the "rotating mass .of plastic in .thegrooves. and that which .forms .the lm between .the lands and: thebarrel.

Thus, as the shortest'vlandineach rset'of stepped lands in the viirstand secondsections of the 1torpedo, .moves past the cylinder -wall in-`its rotation, itleaves .behind ai. thick vcoatingwof :the .iplasticmaterial.' A land Yof intermediateheight follows after the short landandthe mass of'irtating plastic preceding Ysuch intermediate. lland.picks ,up the lcoating' just,.left by Vthe Afshort'; land. The:intermediate `land Jspreads't 'a Csomewhat 'thinner 4nlm .on pthe'cylinderxwalL .whichis vsimilarly pickedxup aridA .mingles Withjheplastic which isfbeingzpushed along .by the third land, having. theleast '..radi'al clearance. 'f '.fThus, mix- ;ing occurs f inyeachrgroove 'due to a' .combination of the previously describedrotation of the contained plastic mass, with the squeezing .or milling'action due to the diminishing. clearance between the bottomvof eachgroove and the bar-rel, the-smearingsaction of the stepped lands; andthepickup by the ,material ingeachzgroove of Vmaterial left lon thecylinder` wallby the :nass-inother grooves. .As the severalgrooves-24f-diseharge their loads into the pockets between-necks -22 thenext'setof groovefsegments, whereithefprocg ess repeated, 1 .andfurther, 1` y mixing occurs, `Because ,theggroovesZ .in eachsection ofthe tor-,- pedo are shallower than those in prior" sections', the linearvelocity of the plastic mass increases as it moves toward the dischargeend of the machine.

When the plastic streams are finally discharged beyond torpedo head 26,their continued right-hand mass revolution and left-hand individualrotation causes further mixing. The mass is then displaced forwardlythrough screen I'I and thence out through extrusion head I5 and itsorice '21. Extrusion occursl substantially without pulsation. Thereafterthe extruded material is handled in the usual manner.

In a. specific embodiment, the illustrated 3- section torpedo wasvconstructed to'iit vin "the .barrel of anfextruder having an internaldiameter of 2.500 inches. The diameter of the highest lands in the firsttwo sections, and of` all lands Ain the final section, was 2.488 inches,leavinga aofg,0.0l6 inch, or a total clearanceof about i3 -mils perinch-of diameter.

.The grooves inthe lfeed Ysection hadaroot diameter of 1.998 inches,

.ora/depth beneath the highest-lands of 0.245

inch. Thosein the center; section had a root di'- diameter of 2.123inches, or a depth beneath the highest lands of .0.183 inch. The grooves:in the `discharge sectionhad a root diameter of 2.248

inches, or a depth of 0.120 inch. The rakev of .the'lands in eachsectionwas that shown on the drawing, -diminishing from inthe feed section tonil in the discharge section.

Flake ethylcellulose was mixed in a--banbury mixer with 16 per cent of.di-tertiaryoctyl diphenyloxide plasticizer and (L5-per Vcent pigment,vand the'resulting dry flakes were `fed through the hopper of anextruder with a sh0rtened feed screw and with the illustrated torpedo,lll/z incheslong, at its-'forward end. The

'plastic was heated in the screw section by means of hot oil circulatingthrough the` jacketof the extruder at 365Y F., and became thoroughlyplastic before entering the torpedo section; .It was extruded as aribbon 3 inches widez and 0.063` inch thick atthe rateof 34 pounds perhour. lExtrusion occurred without pulsation, and the'product wasuniformly colored, showing no striations and being freev from bubblesand other imperfections, showing that-air and other volatile matter hadbeen voided back `through the hopper. Attempts to extrude the samemixture through a conventional screw extruder were unsuccessful, and anextruder having the torpedo of prior Patent No. 2,453,088 gave steadyproduction'of a 'uniformly colored ribbon which, however, had manyincluded bubbles and a rough surface in comparison with the productobtained when using the torpedo' of the present invention. A I claim:1.!'In an extruder, in combination with a screw ifor advancing plasticfeed along a cylindrical barrel: a cylindrical mixingtorpedo beyond thescrew, coaxial and continuous therewith, rotatable within the barrel,and divided into sections by a plurality of circumferentially disposedvyneck-likeA constrictions at intervals along thetorpedo'; a plurality ofevenly spaced `helically` disposed skewed grooves and interven- 'inglands'` of the same hand as the'said screwnand a lead of froml 1/2 to'b2 times the 'length ofthe torpedo, all grooves, inieachlsecture'outside of the torpedo.

vition of the; torpedo being of `equal depth,v but with'those'in each.section afterthe first being shallower lthan thosefin precedingsections,v each` circumferentialfneck being 'of' the depth ofthegrooves' feeding that neck and being bevelled at 'its discharge endvfrom the root diameter-of its feed grooves tothe maximum landdiameter-"of the succeeding section; the lands'in the dischargek sectionof the torpedo all having the same Working clearance from thesurrounding barrel, while those in allprior sections ofthe torpedo arestepped, in the order'of rotation, a

vilrst set comprising a number of 'evenly spaced lands having muchgreater than a minimum Working clearance, succeeding sets of evenlyspaced lands, intervening between those of' the `iirst set, havingsuccessively greater radii, the

last such set of lands having the same radius as all' lands in thedischarge section; the short side yto the leading edge of the succeedingland, having a radius'of curvature approximating theradiusof` thetorpedo and with its centerof curva- 2.- The extrusion apparatus claimedin claim 1, wherein the torpedo is further characterized by havingallofthe lands in the discharge section with a working clearance of about5jmils per inch vof diameter; one-thirdl of the lands in each of theother sections, evenly spaced about the torpedo, of the same height asthose in the discharge section; another third of the lands in said othersections, immediately p'receding'the first third of said lands in orderof rotation, having a clearance of about 9 mils per inch of diameter ofthe torpedo; andthe remaining third of the lands in said other sections,immediately following the'iirstthird of said'lands in order of rotation,Vhaving acleara'nce of about 13 mils per inch' of diameter. n b

3. The extrusion apparatus claimed in claim 1, wherein the torpedo isfurther characterized by being divided longitudinally into threesections, the leading'face of the skewed grooves inthe feed sectionhaving a'rake of about 15 relative to a line normal to the arcuate faceof the groove, while the corresponding angle of rake in the centersection is 'about 6 30' and that in the discharge section is about 0;and, a line normal to the arcuateface ofthe groovesinthe feed sectionforms an angle of about 28 with theradius of the torpedo drawn throughthe midline of the groove opening, while the corresponding angle in thecenter section is about 19 and that in the discharge sectionis about 12v30. FREDERICK E, DULMAGE;

No references cited.

